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| Arthur C. Clarke |
Arthur C. Clarke was a
futurist and writer who is perhaps best known for co-writing the screenplay for
the 1968 film 2001: A Space Odyssey. It
is one of the greatest science fiction films of all time.
After the end of World War
II, Clarke was pursuing his Doctorate in Mathematics and Physics at King's College in London. He joined
the British Interplanetary
Society and soon became the
group’s leader.
In late 1945 he wrote a
paper for his associates about peaceful uses for German V2 rockets. Clarke’s proposed solution was to use the V2’s
to launch geostationary satellites to relay radio communications.
The Society arranged for
Clarke’s paper to appear in Wireless magazine.
It was published in December 1945, 12 years before the actual first earth
satellite.
The article was warmly
received but soon vanished for many years. The successful launch of Sputnik in
1957 by the Soviet Union brought Clarke’s into reality.
Legend has it the
Clarke’s paper was required reading by Soviet scientists.
Today we have Clarke’s entire
Wireless article plus copies of the
original drawings Clarke used to make his points. You can see it online here.
EXTRA-TERRESTRIAL
RELAYS
Can Rocket Stations Give World-wide
Radio Coverage?
By ARTHUR C. CLARKE
ALTHOUGH it is possible,
by a suitable choice of frequencies and routes, to provide telephony circuits
between any two points or regions of the earth for a large part of the time,
long-distance communication is greatly hampered by the peculiarities of the
ionosphere, and there are even occasions when it may be impossible. A true
broadcast service, giving constant field strength at all times over the whole
globe would be invaluable, not to say indispensable, in a world society.
Unsatisfactory though the
telephony and telegraph position is, that of television is far worse, since
ionospheric transmission cannot be employed at all. The service area of a
television station, even on a very good site, is only about a hundred miles
across. To cover a small country such as Great Britain would require a network
of transmitters, connected by coaxial lines, waveguides or VHF relay links.
A recent theoretical study
has shown that such a system would require repeaters at intervals of fifty
miles or less. A system of this kind could provide television coverage, at a
very considerable cost, over the whole of a small country. It would be out of
the question to provide a large continent with such a service, and only the
main centres of population could be included in the network.
The problem is equally
serious when an attempt is made to link television services in different parts
of the globe. A relay chain several thousand miles. long would cost millions,
and transoceanic services would still be impossible. Similar considerations
apply to the provision of wide-band frequency modulation and other services,
such as high-speed facsimile which are by their nature restricted to the
ultra-high-frequencies.
Many may consider the
solution proposed in this discussion too farfetched to be taken very seriously.
Such an attitude is unreasonable, as everything envisaged here is a logical
extension of developments in the last ten years--in particular the perfection
of the long-range rocket of which V2 was the prototype. While this article was
being written, it was announced that the Germans were considering a similar
project, which they believed possible within fifty to a hundred years.
Before proceeding further,
it is necessary to discuss briefly certain fundamental laws of rocket
propulsion and ``astronautics.''
A rocket which achieved a sufficiently great
speed in flight outside the earth's atmosphere would never return.
This
``orbital'' velocity is 8 km per sec. (5 miles per sec), and a rocket which
attained it would become an artificial satellite, circling the world for ever
with no expenditure of power--a second moon, in fact.
The German transatlantic
rocket A10 would have reached more than half this velocity.
It will be possible in a
few more years to build radio controlled rockets which can be steered into such
orbits beyond the limits of the atmosphere and left to broadcast scientific
information back to the earth. A little later, manned rockets will be able to
make similar flights with sufficient excess power to break the orbit and return
to earth.
There are an infinite
number of possible stable orbits, circular and elliptical, in which a rocket
would remain if the initial conditions were correct. The velocity of 8 km/sec.
applies only to the closest possible orbit, one just outside the atmosphere,
and the period of revolution would be about 90 minutes. As the radius of the
orbit increases the velocity decreases, since gravity is diminishing and less
centrifugal force is needed to balance it. Fig. 1 shows this graphically. The
moon, of course, is a particular case and would lie on the curves of Fig. 1 if
they were produced. The proposed German space-stations would have a period of
about four and a half hours.
Fig. 1. Variation of
orbital period and velocity with distance from the centre of the earth.
It will be observed that
one orbit, with a radius of 42,000 km, has a period of exactly 24 hours. A body
in such an orbit, if its plane coincided with that of the earth's equator,
would revolve with the earth and would thus be stationary above the same spot
on the planet. It would remain fixed in the sky of a whole hemisphere and
unlike all other heavenly bodies would neither rise nor set. A body in a
smaller orbit would revolve more quickly than the earth and so would rise in
the west, as indeed happens with the inner moon of Mars.
Using material ferried up
by rockets, it would be possible to construct a "space-station'' in such an
orbit. The station could be provided with living quarters, laboratories and
everything needed for the comfort of its crew, who would be relieved and provisioned
by a regular rocket service. This project might be undertaken for purely
scientific reasons as it would contribute enormously to our knowledge of
astronomy, physics and meteorology. A good deal of literature has already been
written on the subject.
Although such an
undertaking may seem fantastic, it requires for its fulfilment rockets only
twice as fast as those already in the design stage. Since the gravitational
stresses involved in the structure are negligible, only the very lightest
materials would be necessary and the station could be as large as required.
Let us now suppose that
such a station were built in this orbit. It could be provided with receiving
and transmitting equipment (the problem of power will be discussed later) and
could act as a repeater to relay. transmissions between any two points on the
hemisphere beneath, using any frequency which will penetrate the ionosphere.
If
directive arrays were used, the power requirements would be very small, as
direct line of sight transmission would be used. There is the further important
point that arrays on the earth, once set up, could remain fixed indefinitely.
Moreover, a transmission
received from any point on the hemisphere could be broadcast to the whole of
the visible face of the globe, and thus. the requirements of all possible
services would be met (Fig. 2).
Fig. 2. Typical
extra-terrestrial relay services. Transmission from A being relayed to point B
and area C; transmission from D being relayed to whole hemisphere.
It may be argued that we
have as yet no direct evidence of radio waves passing between the surface of
the earth and outer space; all we can say with certainty is that the shorter
wavelengths are not reflected back to the earth. Direct evidence of field strength
above the earth's atmosphere could be obtained by V2 rocket technique, and it
is to be hoped that someone will do something about this soon as there must be
quite a surplus stock somewhere!
Alternatively,' given
sufficient transmitting power, we might obtain the necessary evidence by
exploring for echoes from the moon. In the meantime we have visual evidence
that frequencies at the optical end of the spectrum pass through with little
absorption except at certain frequencies at which resonance effects occur.
Medium high frequencies go through the E layer twice to be reflected from the F
layer and echoes have been received from meteors in or above the F layer. It
seems fairly certain that frequencies from, say, 50 Mc/s to 100,000 Mc/s could
be used without undue absorption in the atmosphere or the ionosphere.
A single station could
only provide coverage to half the globe, and for a world service three would be
required, though more could be readily utilised. Fig. 3 shows the simplest
arrangement. The stations would be arranged approximately equidistantly around
the earth, and the following longitudes appear to be suitable :--
|
30
|
E
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-- Africa and Europe.
|
|
150
|
E
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-- China and Oceana.
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|
90
|
W
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-- The Americas.
|
Fig 3. Three satellite
stations would ensure
complete coverage of the globe.
The stations in the chain
would be linked by radio or optical beams, and thus any conceivable beam or
broadcast service could be provided.
The technical problems
involved in the design of such stations are extremely interesting, but
only a few can be gone into here. Batteries of parabolic reflectors would be
provided, of apertures depending on the frequencies employed. Assuming the use
of 3,000 Mc/s waves, mirrors about a metre across would beam almost all the power
on to the earth. Larger reflectors could be used to illuminate single countries
or regions for the more restricted services, with consequent economy of power.
On the higher frequencies
it is not difficult to produce beams less than a degree in width, and, as
mentioned before, there would be no physical limitations on the size of the
mirrors. (From the space station, the disc of the earth would be a little over
17 degrees across). The same mirrors could be used for many different
transmissions if precautions were taken to avoid cross modulation.
It is clear from the
nature of the system that the power needed will be much less than that required
for any other arrangement, since all the energy radiated can be uniformly
distributed over the service area, and none is wasted. An approximate estimate
of the power required for the broadcast service from a single station can be
made as follows : -- The field strength in the equatorial plane of a /2 dipole in free space at a distance of d metres
is 4
|
e = 6.85
|
__
\/ P -- d |
volts/metre,
|
where P is the power
radiated in watts.
|
Taking d as 42,000 km
(effectively it would be less), we have P = 37.6 e 2 watts. (e now in V/metre.)
If we assume e to be 50
microvolts/metre, which is the F.C.C. standard for frequency modulation, P will
be 94 kW. This is the power required for a single dipole, and not an array
which would concentrate all the power on the earth. Such an array would have a
gain over a simple dipole of about 80. The power required for the broadcast service
would thus be about 1.2 kW.
Ridiculously small though
it is, this figure is probably much too generous. Small parabolas about a foot
in diameter would be used for receiving at the earth end and would give a very
good signal noise ratio. There would be very little interference, partly
because of the frequency used and partly because the mirrors would be pointing
towards the sky which could contain no other source of signal. A field strength
of. 10 microvolts/metre might well be ample, and this would require a
transmitter output of only 50 watts.
When it is remembered that
these figures relate to the broadcast service, the efficiency of the system
will be realised. The point-to-point beam transmissions might need powers of
only 10 watts or so. These figures, of course, would need correction for
ionospheric and atmospheric absorption, but that would be quite small over most
of the band. The slight falling off in field strength due to this cause towards
the edge of the service area could be readily corrected by a non-uniform
radiator.
The efficiency of the
system is strikingly revealed when we consider that the London Television
service required about 3 kW average power for an area less than fifty
miles in radius.
A second fundamental
problem is the provision of electrical energy to run the large number of
transmitters required for the different services. In space beyond the
atmosphere, a square metre normal to the solar radiation intercepts
1.35 kW of energy. 6 Solar engines have already been devised for
terrestrial use and are an economic proposition in tropical countries. They
employ mirrors to concentrate sunlight on the boiler of a. low-pressure steam
engine. Although this arrangement is not very efficient it could be made much
more so in space where the operating components are in a vacuum, the radiation
is intense and continuous, and the low-temperature end of the cycle could be
not far from absolute zero. Thermo-electric and photoelectric developments may
make it possible to utilise the solar energy more directly.
Though there is no limit
to the size of the mirrors that could be built, one fifty metres in radius
would intercept over 10,000 kW and at least a quarter of this energy
should be available for use.
Fig. 4. Solar radiation
would be cut off for a short period
each day at the equinoxes.
The station would be in
continuous sunlight except for some weeks around the equinoxes, when it would
enter the earth's shadow for a few minutes every day. Fig. 4 shows the state of
affairs during the eclipse period. For this calculation, it is legitimate to
consider the earth as fixed and the sun as moving round it. The station would
graze the earth's shadow at A, on the last day in February. Every day, as it
made its diurnal revolution, it would cut more deeply into the shadow,
undergoing its period of maximum eclipse on March 21st. on that day it would
only be in darkness for 1 hour 9 minutes.
From then onwards the
period of eclipse would shorten, and after April 11th (B) the station would be
in continuous sunlight again until the same thing happened six months later at
the autumn equinox, between September 12th and October 14th. The total period
of darkness would be about two days per year, and as the longest period of
eclipse would be little more than an hour there should be no difficulty in
storing enough power for an uninterrupted service.
Conclusion
Briefly summarised, the
advantages of the space station are as follows:--
(1) It is the only way in
which true world coverage can be achieved for all possible types of service.
(2) It permits
unrestricted use of a band at least 100,000 Mc/s wide, and with the use of
beams an almost unlimited number of channels would be available.
(3) The power requirements
are extremely small since the efficiency of ``illumination'' will be almost 100
per cent. Moreover, the cost of the power would be very low.
(4) However great the
initial expense, it would only be a fraction of that required for the world
networks replaced, and the running costs would be incomparably less.
Epilogue--Atomic Power
The advent of atomic power
has at one bound brought space travel half a century nearer. It seems unlikely
that we will have to wait as much as twenty years before atomic-powered rockets
are developed, and such rockets could reach even the remoter planets with a
fantastically small fuel/mass ratio --only a few per cent. The equations
developed in the appendix still hold, but v will be increased by a factor of
about - a thousand.
In view of these facts, it
appears hardly worth while to expend much effort on the building of
long-distance relay chains. Even the local networks which will soon be under
construction may have a working life of only 20-30 years.
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